Tuesday, December 10, 2019

Alliances in the Oil Field

In the oil field, two factors drive profits. The first, market price of oil or gas, is governed by many elements, such political stability, economic growth and the weather, all of which aare outside control of operators. However, the second factor, production cost, can be controlled to some degree by the industry. 


During the past decade, market price has stabilized - albeit at a moderate level - but production costs continue to increase. Wells cost more to drill and bring on stream because much of the easy oil is gone, leaving behind oil that defies production by conventional techniques and oil in deeper, more complex reservoirs in frontier areas.

Total production costs remain high because productivity per well has declined and the techniques and material required are generally more expensive.

Striving to remain profitable, oil companies are taking action in two areas to control costs. First, they are redefining their business, identifying core competencies and outsourcing noncore activities. Second, they are changing the way they do business, gradually converting the arm's-length relationship with contractors into more cooperative collaborations to eliminate redundancy and boost efficiency, exploiting new technologies to enhance productivity.

Oilfield business relationships take many forms. Volume discounts, turnkeys, service bundling, integrated services, joint ventures, partnerships, alliances- each has a place in the continuum of business practice, each with different levels of cooperation and  trust. Volume discounts and turnkeys are variations on the traditional way of doing business. Jobs are bid, whether by well or by project, and job spesifications are set by the operator. The service company reacts, then execute the job on demand. 

In a second category, service bundling and integrated services are new ways of doing business that are gaining acceptance, especially outside North America. Service bundling gathers several services under one contract and concentrates the points of contact between the operator and contractors. Here, the operator still provides all the specs, and the service supplier executes the job. Integrated services contracts span a wide range of activities, from service execution- performing bundled services - at the most basic end, to product delivery at the most sophisticated end. Product delivery, in which the product may be an offshore platform, a well or some other complicated project, entails conceptual design, process planning, service execution and evaluation. 

Joint ventures tend to denote shared equity and sometimes result in acquisition of one party by the other. 

The third category, and perhaps the newest in the oil industry - certainly the hardest to define- includes partnerships and alliances. Partnerships are defined by the Journal of Petroleum Technology as "short-term, project spesific relationships between supplier and client that seek to gain greater economic value for both parties. Alliances are similar to partnerships, except they are designed to persist beyond the scope of individual projects. Other definitions exist, but an alliance is defined here as a long-term relationship between two companies that furthers their common interests over a specific range of activities. 




 Efficiency Improvements through Alliances

The cooperative spirit of an alliance changes the way problems are approached. In the quest to cut costs, it means no dwelling on contractor profit, but cutting total project cost. To uncover where cuts can be made, every process in the entire project must be analyzed and examined for inefficiencies. Alliance partners construct a description, called a process map, for each process.









Sunday, November 17, 2019

Reducing 3D Seismic Turnaround

There are two main reasons oil and gas producers worry about time spent on 3D seismic acquisition and processing, called turnaround time. First, in the oil and gas business, as in every business, time is money. The more time spent on drilling, logging and well completion, the longer the delay in production and the lower the profit. 

Add the time to acquire and interpret seismic data before drilling, and the delay in bringing reserves to surface may grow beyond the schedules and budgets of many production managers.

Second, and special to the oil and gas business, saving time can make the difference between being able to do business and not. Development contracts worldwide require oil companies to drill within a specified time. The clock starts ticking once acreage is licensed. A 3D seismic survey planned, acquired, processed and interpreted in advance arms developers with tools for intelligent well placement, yielding higher production from fewer wells. 

More 3D seismic surveys are also being commisioned for exploration, in addition to field development, their initial application. Unlike 2D seismic, which grew from the exploration market into development, 3D seismic has grown in the opposite direction. Companies are discovering that early acquisition of 3D data reduces finding costs and overal project costs. Interpreted seismic data are essential for intelligent bidding on acreage. And some exploration contracts now require a 3D survey before drilling. This expansion into exploration, along with decreases in the cost of seismic acquisition and processing, has raised demand for 3D seismic data .

This increased demand has forced service companies to reduce turnaround time- without sacrificing quality. This article looks first at the dramatic improvements in marine turnaround time, then at the steps being takento significantly reduce turnaround in transition zone and land surveys.


The Marine Story

Three years ago, a marine survey of 500 km square took a year or more to be acquired and processed. Today, through a combination of new technologies, turnaround time for similar surveys can be as little as nine weeks. Technolgies responsible for this dramatic reduction vary from faster acquisition capacity to high speed links with shore-based computers for real-time, full-scale processing.

Today seismic vessels can acquire data 12 times faster than they could in the early 1980s, thanks to multielement acquisition- multiple air gun sources, multiple receiver streamers and evel multiple vessels. Prior to 1984, vessels towed one source array and one 3-km streamer. This configuration evolved to two streamers and two sources per vessel by 1986, quadrupling the area covered with each traverse, and decreasing the cost per unit area. In 1990, streamer length started to increase, also decreasing costs. By 1991, there were two sources firing alternately to three streamers, and by 1992, there were four streamers. And, in a continuing quest for greater capacity, contractors are now building or refurbishing seismic vessels to tow 8 to 12 streamers.




A challenge in designing vessels for multi-streamer acquisition is to keep all the streamers uniformly separated while maintaining vessel speed. Streamers are separated with a deflector, which steers outer streamers away from their normal stream lines. Most streamers follow angled slabs-paravenes- which deflect the streamer outward, but also create drag on the vessel. Each 3-km deflected streamer may exert up to 12 tons of drag, forcing the vessel to consume more fuel to maintain speed. Eight to twelve streamers, with paravenes deflecting the outer ones, would act like a sea anchor, creating enough drag to stop an ordinary vessel. One contractor, PGS Exploration , is designing a more powerful vessel to address this problem.

Rather than design a larger, more expensive vessel to tow more streamers, GecoPrakla has designed the monowing deflector. Acting like an airplane wing flying through water, this "lifts" the streamer apart, and result in a 500% increase in lift-to-drag ratio compared to conventional deflectors. The reduced drag increases acquisition efficiency, and also safety. The lower tension in the lead-in, or tow cables, between the vessel and the streamers, reduce the chance of a tow cable snapping and flapping back to hit the vessel. And unlike other deflectors, orientation of the Monowing can be controlled remotoely, to act as a rudder for the streamer. This allows streamer spacing to be controlled from the vessel, and permits individual streamers to be spooled in for repairs.  

The Monowing deflector has already been deployed in the Irish Sea and West Africa, to tow six streamers. It is being tested with five streamers at extra-wide 150-m spacing, making the 600-m swath acquired in a single vessel pass the widest ever. 

Streamers themselves have also been upgrade. In earlier, analog streamers, hydrophones were wired to the streamer cables and the analog signal transmitted up the streamer and then digitized.  

There may have been signal leakage in the streamer, or cross-talk, in which a signal from one hydrophone gets mixed with that from another. With digital streamers, the signal is recorded digitally so cross-talk is eliminated. Digital streamers are also more reliable, resulting in less downtime and better turnaround.



 While multielement acquisition has played the leading role in reducing acquisition time, it has created a new challenge in reducing overall turnaround time. Data can arrive at a staggering 5 MBytes/sec and some of it must be processed before the next shot is fired- about every 10 seconds- if the processing is to keep pace.

Rising to the challenge is concurrent processing, a combination of onboard processing and high-speed communication with onshore computers and decision makers.

To achieve minimum turnaround time, two sets of data- source signature quality and survey position - must be processed between shots. The source is a cluster of different-sized air guns. On Geco-Prakla vessels the air guns are controlled by the integrated acquisition and processing system. This module fires the air guns in a sequence that is tuned to their sizes. As the size of the gun increases, so does the time from firing to maximum pressure. The controller synchronizes the guns' pressure maxima, giving a stronger source signal. 

The hardware also monitors source output to check the quality of each shot.  

The sensors, located within one meter of the air guns, communicate with the vessel through fiber-optic connections, and are packaged based on concepts from Anadrill's measurements-while-drilling (MWD) technology. In this hostile environment, near a high-energy source and sustaining at least 500,000 shocks per year, the rugged construction that ensures reliable MWD also helps reduce seismic turnaround.

To maximize vessel uptime, errors such as a gun going off at the wrong time, or not at all, must be detected immediately. Then processing specialists can determine whether the shot must be retaken, or whether the recorded signal satisfies the geophysical objectives of the survey. If the signal is sufficient, time is saved. If insufficient, time is still saved, because a seismic line can be quickly reshot while the vessel is still over the survey area.

The second set of data that must be processed between shots is survey position coordinates, called navigation data. Navigation data describe the position on the earh of every source and receiver point in the 3D survey. The data come from relative position measurements made with every shot as the vessel is in motion. The position of the vessel relative to satellites is determined using the Global Positioning System (GPS). The in-sea positions of the seismic sources and receivers are computed using directions from compasses mounted on the streamers and distance information-ranges-provided by acoustic sensors and lasers distributed in networks across the ends of the streamers.  




The TRINAV module of the TRILOGY system collects the compass, laer and acoustic signals, detects transit times, processes them for range , computes the network node positions, calculate source and receiver positions and stores the results in a data base before the next shot is fired. 

The number of sensor data measurements- including compass data, laser ranges and bearings , satellite and radio position signals - used in such a calculation has grown from 15 in the days of single source and single streamer, to more than 350 now with dual sources and eight streamers. 

Checking that the positions fall within the project spesifications is a daunting task, and one whose automation has further reduced turnaround time. Until recently, this was done subjectively by navigation analysts, visually checking plots and position listings. Now, computed positions are quality assured using position acceptance criteria (PAC), automating the time-consuming task and slashing weeks of turnaround. The PAC are established by comparing the range in question to the range of the last shot. If the two are within a predefined threshold, the range is accepted. Deviations are flagged by the computer, making them easy to spot.

While navigation data are being collected and processed, the seismic traces are beginning their journey through data processing. Essentially any processing offered by onshore processing centers can be supplied onboard. 

A Turnaround Breakthrough

In the summer of 1994, Statoil, in partnership with Saga and Mobil, conducted a 3D turnaround pilot project  in block 33/6 of the Norwegian North Sea. The area had already been traversed with 2D lines. The acreage covered in the 3D survey was an extension of play concept that had proven prolific to the south - the oil basin contains the Statfjord field, estimated at more than 3.5 billion barrels of recoverable oil, and the Snorre field. 



 The 33/6 area will be part of concession round 15, recently announced by the Norwegian government. With this survey already acquired, processed and interpreted, the oil companies, acting individually, can make better decisions about how to bid for acreage. 

The goal of the pilot project was to turn around the 313-km square survey in seven weeks. With conventional technology, such a survey would take 18 weeks: 6 for acquisition, then at least another 12 for processing. Executing such a tightly constrained survey requires exact planning. Survey design, acquisition parameter selection and choice of processing chain were given special attention by Statoil and Geco-Prakla geophysicists. In addition to these standard steps, during the planning phase it was recognized that to minimize turnaround time, both Statoil and Geco-Prakla would have to reevaluate accepted working practices: Statoil agreed to hold decision-response time to 12 hours, and Geco-Prakla agreed to increase computer and communication resources that would allow more rapid acquisition and processing.

The Geco-Prakla vessel , Geco Gamma, was equipped with the latest technology for the job. Gamma had the TRILOGY system for onboard navigation and seismic data processing, and access to INMARSAT, the international marinet satellite system. Three IBM RISC 6000s were installed to handle the near real-time processing, reproducing the software and hardware of an onshore processing center. The data would travel directly from the acquisition system to the memory of the TRIPRO onboard processing system. The plan called for crucial data to be transmitted via satellite and land lines to the Statoil office in Stavanger, Norway, where a workstation was installed with the same processing and interpretation software. 









The first shot was fired on June, 22, 1994, with the vessel towing two air gun clusters and four 3000-m streamers spaced 75 m apart. The survey was 11 km wide and was completed in 38 vessel passes, making 293 lines. Some of the first lines were shot in bad weather, which created low-frequency swell noise, above the tolerance level set in the presurvey plan. When that level is exceeded, many oil companies choose to shut down acquisition, and the vessel stands by, at up to $30,000 per day, waiting for weather to calm. But onboard processing showed that the noise could be filtered out, though the filtering would have to be done prestack. 

By monitoring signal quality onboard, and processing the acquired, subspecification data in real time, Geco-Prakla geophysicist were able to decide that the processing scheme would tolerate the noisier data. This eliminated the need to reshoot five or six lines, saving $70,000. The savings paid for the added cost of equipping the vessel with the RISC 6000s, and cut two days off the turnaround.

Early in the planning, the team considered undertaking onboard processing of reduced fold data. But test conducted prior to acquisition indicated that the reduced fold would give inadequate imaging of subsurface reflectors, so full, 30-fold data were processed onboard.

One of the crucial phases of the survey was the construction of the earth velocity  model that would be used to stack and later to migrate the data.



Geco-Prakla geophysicist analyzed velocities on 18 seismic lines selected at 500-m intervals, and transmitted their results via satellite to Stavanger.


 












Statoil geophysicist loaded the data on workstations in their offices and worked weekends to monitor data quality and relay decisions on the quality of the velocity picks back to the vessel. A velocity model for the 3D volume was then built onboard.


The last major step before stacking- 3D dip moveout processing (DMO)- was also completed onboard for the 30-fold data. This process corrects for the reflection point smear that results when events from dipping reflectors are stacked. The final stack volume was being built as soon as the last shot was fired, and inline migration begun while the vessel was steaming back to port.

The computers and processing specialists were flown to Stavanger, where the final processing was completed three weeks later. Data quality was equivalent to that of a normal onshore processing job, and no immediate reprocessing was scheduled. Seven weeks after the first shot was fired, a Charisma workstation-ready tape was produced, waiting to be interpreted .






Fastracks and Quicklooks


Reduced-turnaround surveys are evolving rapidly, and the amount of processing that goes into each survey varies. Specialists divide reduced-turnaround surveys into two categories : fastracks and quicklooks. Fastracks are fast, fully processed surveys, like Statoil's 33/6. Quicklooks are surveys that process a subset of the full data set- called low-fold- or that simplify processing, such as skipping dip moveout processing. 

Quicklooks give interpreters a head start on interpretation, allowing earlier exploration or development decisions and identifing areas that deserve more detailed processing. BP Exploration has conducted four such surveys offshore Vietnam with Geco-Prakla, using onboard processing of navigation, low-fold data and widely spaced streamers to speed turnaround. In one case, BP had farmed into a prospect- taken over a license relinquished by another operator- with only two years remaining. At the time, the planned 3D survey would have taken six months for full-fold processing, compared to 11 weeks for a low-fold interim data cube. By getting the data earlier, BP interpreters were able to spend more time understanding the prospect before the spud date deadline. 

Quicklooks can be considered preliminary or intermediate results, with potential to benefit from later reprocessing. One example is a 700 km square exploration survey shot and processed onbard by Geco Resolution for Mobil in Papua New Guinea. Only portions of the survey were processed with full fold,saving some of the exploration money for drilling and development.  

Today, quicklooks and fasttracks alike are possible only if the onboard processing sequence is nearly set in stone during presurvey planning with tests on prior 2D data. If acquisition conditions require procesing modifications, some, such as noise attenuation, can be accomodated during the survey.

The Onshore Challenge

Today, turnaround for 3D land and TZ surveys can be only unfairly compared with that for marine surveys. The main difference is in acquisition, which in some cases may take 50 times longer on land than that at sea.

There is also litte formal data on the trends in turnaround for land and TZ surveys, because no two surveys can be  compared. In the relatively constant marine environment, where every survey has roughly the same sources, receivers, subsurface and acquisition geometry, surveys of different sizes and from different areas can be scaled up or down for the purposes of keeping statistics. 

However, on and near land, every survey is different, and turnaround comparisons from one area to another may be meaningless. The environment may vary from swamp to arctic tundra, from desert to jungle. Sources, receivers and acquisition geometries come in as many combinations as there are environments. But in spite of the absence of statistics, land and TZ turnaround are improving. 

Paralleling improvements in marine turnaround, TZ and land surveys are seeing more reliable acquisition hardware, faster acquisition through multiple sources and more receivers, and real-time verification of source and receiver positions. The following two sections describe case studies - first transition zone, then land - to demonstrate some of the latest techniques to shorten turnaround.

Transition Zone

The North Freshwater Bayou in southern Louisiana, USA, was the site of a 3D survey demanding exceptional turnaround. The acreage covered leases operated by Unocal and Exxon. Unocal was drilling at the time of the survey, and planned at least one additional well. Drillers, heading for a deep target below 4.0 sec two-way travel time, wanted to confirm the location of the target before reaching total depth. The challenge was to complete acquisition between July 15 end of the alligator breeding season and the October 15 start of duck migration- a 13-week window of opportunity.

Survey planners designed a 200 km square to be processed in two phases. Processing began on an 46 km square priority area, while acquisition continued over surrounding acreage.

The shallow water environment allowed an all-hydrophone acquisition. Some TZ surveys cross the line between water and land, and require a combination of receivers - geophones on land and hydrophones in the water. Processing such surveys takes extra steps to account for the different responses of the various receiver types.

The hydrophones used in the North Freshwater Bayou were attached to the Digiseis FLX system, a new, flexible transition zone acquisition system developed by Geco Prakla. Each Digiseis-FLX data acquisition unit (DAU) is a floating instrumented tube, tethered to an achor and connected to four hydrophone groups. Up to 1536 channels have been recorded in real time without reaching the limit of the system. This large number of channels allows for flexibility in arranging source-receiver combinations, often without moving the DAU. Seismic data are transmitted to the acquisition boat using radio frequencies that can be adapted to avoid conflict with other radio activity. 

The Digiseis-FLX system presents advantages over other TZ equipment, called bay cable. Bay cable consists of a 1/3-in. diameter instrumented cable, two to three miles long, that lies on the sea bottom. 

The cable can shift with currents, and can be damaged by boat propellers and sharp coral. While radiotelemetry avoids these problems, the added flexibility creates a new problem, synchronization: each unit must record at exactly the same time. The Digiseis-FLX system uses a patented synchronization method, achieving an accuracy significantly higher than other radiotelemetry systems. 

Another innovation that contributes to the speed of the survey is the method with which the source explosives and the hydrophones are emplaced. The technique- ramming- is like using a hypodermic needle to inject a source or receiver into the earth. Ramming sources in soft transition zone cuts down on the time required to drill source holes. On land, drilling crews typically drill 100 to 180-ft [30 to 55 m] deep shot holes in advance of the acquisition crew. Equivalent results are obtained with 40 to 50 ft deep ram holes. 

Ramming not only takes less time, but it also costs less. Deep holes cost about $300 per hole to drill, while ramming cost about $75 per hole. Ramming hydrophones to a uniform depth of 20 ft [ 6 m] below sea level results in better receiver coupling and higher quality data. The main limitation of ramming is the restriction to unconsolidated earth.

Not all the North Freshwater Bayou turnaround speed came from fast acquisition. Geometry verification- much like navigation data processing in the marine environment- carried out in the field, cut weeks off the normal processing time. Geometry verification, a feature of the Voyager mobile data processing system, checks that the source and receiver positions attributed to every shot record are correct. Usually this is checked back at the office after acquisition has been completed and the crew has left, but fixing errors after the fact is time-consuming. In some cases, entire land surveys have had to be reshot- a turnaround nightmare.

One error typically encountered in geometry verification is a mistake in the identifcation of shot-point location. This can occur when the source, say a vibrating truck (vibro for short) is at the wrong location, can't get to the right location, or if the location is miss-surveyed. It can also occur if receiver locations are missurveyed, or if the wrong receivers are active.

These mistakes can be detected quickly by applying some simple processing at the base camp, after the day acquisition. The process is called linear moveout , or LMO. LMO compares arrival times recorded for a given source-receiver geometry to those expected for the same geometry, assuming a constant velocity subsurface. If the source and receivers are in the right places, the LMO process yields seismic traces with first arrivals aligned in time. Any other pattern of first arrivals indicates a mistakes in the source-receiver geometry. 

This technique was used in the Unocal survey to quickly verify geometry in the field. Catching errors with the crew still on site permits corrective action. Shot and receiver locations can be resurveyed if necessary to revise the location data base. Without this field verification, errors may be detected weeks or months later. Then, processing specialists would have to test several possible geometries in hopes of discovering what really happened, spending time and adding uncertainty. Verifying the geometry in the field saves up to four weeks in the office. 

With much of the time-consuming work out of the way, the computing center proceeded with the rapid disk-to-disk processing on a Sun SPARCstation 20. The fully processed 3D cube was ready three weeks after acquisition, in time for interpreters to use. 

Interpretation of the seismic volume signaled drillers that their target would be productive. Unocal interpreters were able to use the seismic data to confirm the quality of their next well location and plan at least one additional deep well at greater than 20,000 ft [6090 m].

Reducing Turnaround on Land

Three-dimensional surveys on land encounter many of the same difficulties as in transition zones, with the added problems of access, topography and extreme temperatures. All of these make for longer acquisition campaigns and more difficult processing.  Under fair marine conditions, multielement acquisition can collect more than 75 km square per day. Under extreme land conditions, such as -40 degree C arctic surveys, acquisition may proceed at less than 1 km square per day. Land surveys of 1500 km square have taken up to 4 1/2 years for acquisition. 

In land surveys more than other types, presurvey planning is the key to minimizing turnaround. Time spent planning and designing is more than compensated by time saved acquiring data. With a given set of equipment, say a certain number of geophones and people, one plan might achieve 150 to 200 shots a day, while suboptimal plan with different shot and receiver line spacing may collect only 100 shots a day.

The most time-consuming tasks in acquisition - be they laying out receivers, drilling shot holes, repairing damaged cables or advancing to the next vibro location - must be identified and minimized to reduce turnaround. In the following examples of 3D land surveys in Texas, such bottlenecks were identified during presurvey planning and circumvented in novel ways. 

Rough Terrain Turnaround

The Val Verde basin in Texas, USA is at the edge of the Sierra Madre mountains that extend north from Mexico. The basin is a hot play for gas, with some wells in the region producing more than 7 MMcf/D. The terrain is extremely rough, with steep-edged mesas and incised canyons. Several 3D surveys in the area have contributed to the continuous improvement of field operating procedures.

In one case, Conoco joined forces with Hunt Oil to acquire the Geaslin survey in the summer of 1994. Both companies had a short fuse: they had to evaluate their leases and make decisions for an early 1995 drill date. The survey design specified the number and location of shot points, but the short turnaround and high cost ruled out dynamite as a source, because too much time would be taken to drill shot holes. 

Vibro sources were available- four vibrating trucks at 12.5-m spacing constitute one source - but the terrain presented mind-boggling logistics: in some cases it would take four hours for a vibro trip up and down a mesa.  

The solution was to use two sets of buggy vibros, or eight in all, similar to a dual-source marine survey. While one set was shaking in the valley , the other set would work its way up a mesa. Similar dual-source vibro operations have been extremely successful in desert areas, such as Egypt and Oman, where there are no obstructions. In this case they allowed acquisition of 60 sq miles [153 km square] in 65 days.

As in all land jobs, darkness presents too many hazards, so the crew operates only during daylight hours. Evenings were well spent, though, running geometry verification on the day's acquired data. One of the goals of the next shift was to have that day's geometry checked and attached to the seismic traces, usually by midnight. That way, geometry problems could be fixed the next day, before the receivers were moved. 

Processing the data from the Geaslin survey proved to be a great challenge. Val Verde basin is notorous for bad data. High velocity carbonates near the surface deflect much of the source energy away from deeper layers; receiver and source coupling to the surface varies with location; and the rugged relief introduces high residual statics- differences in seismic travel time through surface topography. After four months of testing and processing, including 3D DMO and migration, the processing was complete. The next step is preparation, in preparation for a possible 1995 drill date. 

In the nearby Brown Basset survey for Mobil, acquisition time was further shortened by the use of helicopters to move cables, recording boxes and geophones up and down the mesa and canyon walls. Three hundred "helibags" - net bags for transporting material- helped the crew complete the 153 km square acquisition in significantly less time than usual. 

What's Coming to Land

Keeping track of all the information pertinent to a land survey is often the most time consuming job, and steps are being taken to shorten it and make fuller use of all the information available. 

The Olympus-IMS information management system, now in use by Geco-Prakla in Germany, is designed to do just that.

The Olympus-IMS system colocates in a single data base the many types of data that must be handled in a land survey. Previously, every type of data had its own data base: the planned survey layout, the actual surveyed receiver and source point locations, shot hole drilling data, shooting schedule data and the recorded seismic trace data were handled by different software. The new integrated system minimizes the number of data handling steps, reducing errors and improving turnaround. The system will also link directly with processing software to allow field processing for geometry verification and further processing steps. 

Further improvements in land turnaround will come from improvements in hardware and communication. In the most adverse conditions, a good crew may spend as little as two to three hours shooting out of ten spent in the field. In these circumstances,a small amount of time spent trouble-shooting equipment faults can have a considerable impact on turnaround. Geco-Prakla engineers are developing more reliable hardware, to reduce the amount of time spent looking for and repairing flaws in geophones, cables and connectors. Today, each receiver point marked on a map consists of up to 72 individual geophones, whose signals are combined to yield a less noisy signal at a central location, or source point. Up to 140,000 geophones will have to be repeatedly picked up, put down and maintained in the course of a 3D survey. Efforts are also underway to find new ways to acquire the same amount and quality of data with fewer receivers, cutting survey time. 

Improved communications will also cut turnaround time. Increased use of GPS is decreasing the time spent  surveying positions for land source and receiver points. Surveying with GPS is faster and easier to check than traditional theodolitic surveying, and leaves less room for human error.  Placing GPS units on vibro sources helps keep track of actual source locations and reduces location error.

For arctic land surveys, snow streamers have been developed in collaboration with Norsk Hydro as substitutes for hand-placed geophones in an effort to increase acquisition efficiency. Geco-Prakla engineers have tested snow streamers in six programs, acquiring 1200 km of 2D data. Efforts are also underway to minimize environmental impact, which in arctic environments must be included as part of turnaround- a single drop of oil spilled must be recovered before the crew moves.

Connecting land crews via satellite to SINet, the Schlumberger Information Network, will give better day-to-day contact with office bases, speeding equipment and supply requests and allowing interaction with processing centers. 

Moving more processing to the field will further reduce turnaround for both land and transition zone surveys. Parameter testing, noise attenuation and velocity picking can be done with today's field processing tools. But full concurrent processing, as performed in marine surveys, is still a dream for land. 

Land acquisition, more so than marine , is a three-dimensional problem: sources are not aligned with receiver lines, and more time is needed to acquire enough seismic traces to process one part of the 3D volume. 

At best, processing through to stacking could lag acquisition by a few weeks, but the difficult task of computing residual statics before stacking cannot begin until all data are in. Advances may come from taking a new view of 3D land surveys- planning, acquiring and processing with a truly three-dimensional view- rather than simply repeating a series of two-dimensional snapshots.

The Role of Integrated Services in Reducing Turnaround

Marine, TZ and land 3D surveys are sure to find further turnaround improvement in the common ground of integrated services. In an integrated-service survey, planning, acquisition, processing and project management are delivered by one service company. Traditionally, the oil company plans the survey, then one contractor acquires the data and another processes it. Time is wasted transferring data and responsibility between parties. 

Geco-Prakla has developed an integrated service for 3D surveys called TQ3D- Total Quality 3D. Larger in area than most surveys, TQ3D projects can cover leased and open blocks. A TQ3D projects may be operated from 100% nonexclusive, or anywhere in between. Data acquired on proprietary basis become the property of the operator. 





 























































Tuesday, October 29, 2019

Superdeep Borehole into The Earth

The drill bit has stopped turning and the KTB project is winding down. Germany's superdeep borehole is complete. How and why was it drilled? And what have the scientist achieved so far?

 Thermal gradients, heat production, stress fields, fluid transport, deep seismics and deep resistivity are all of great interest to earth scientist. Studying these fundamental topics helps them unravel the mysteries of weather fluctuations, the distribution of mineral resources, and natural disasters such as earthquakes, volcanoes and floods. Rock outcrops, river gorges and cliff faces provide visual evidence to interpret deep probing measurements such as seismics, magnetics and gravimetrics. Commercial mining and drilling have also guided scientist , giving tangible connections to surface observations. However, drilling has been used specifically for scientific research only within the last thirty years.

The internationally funded Ocean Drilling Program (ODP) was started as part of a worldwide effort to research the hard outer layer of the Earth's crust called the lithosphere. Results from this project have been dramatic, providing real evidence of continental drift and plate tectonics. The lithosphere is made up of six major and several minor rigid moving plates. New oceanic crust is formed and spreads out at mid-ocean ridges and is consumed at active plate margins-subduction zones- where it sinks back into the Earth's mantle. This process takes up to a few hundred million years. 

Continents are different. They are made of lighter rock and are not easily recycled, allowing them to achieve ages of 4 billion years. They also provide the vast majority of the world's resources , so it is vital to understand their structure and development. One way of doing this is to extend the work started by ODP to the continent. KTB- which stands for Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland, or German Continental Deep Drilling Program- is drilling one of a handful of borehole specifically for continental scientific research. This article looks at the major drilling achievements of KTB, at the Schlumberger wireline logging contribution and at some of the main areas of research. 

The project was initiated in 1978 by a working group of the German Geoscientific Commision of the German Science Foundation. The group discussed more than 40 possible drillsites in Germany, eliminating all but those with the broadest possible research potential. Two sites were chosen for further studies: Haslach in the Black Forrest region of South Germany and Windischeschenbach 80 km east of Nurnberg in Bavaria, southeast Germany. In 1985, the Federal Ministry for Research and Technology gave the final approval for the KTB deep drilling program and both sites were comprehensively surveyed. 

Both geology and the expectation of a lower formation temperature gradient favored the Windischescenbach site. The site is located on the western flank of the Bohemian Massif about 4 km  east of a major fault system- the Franconian line.Scientists also believe it lies at the boundary of two major tectono-stratigraphic units in Central Europe- the Saxothuringian and Moldanbuian. This boundary- which they hoped to cross - is regarded as a suture zone formed by closure of a former oceanic basin 320 million years ago. This process gave rise to a continent-continent collision- forming a mountain chain and the present day Eurasian plate. The mountains have long since eroded away, exposing rocks that were once deeply buried. Therefore, this area is ideal for the study of deep-seated crustal processes. In addition, geophysical surface experiments have shown that the area around the drillsite has unusually high electrical conductivity and strong gravimetric and magnetic anomalies, which deserve closer investigation.


The scientific challenges for the KTB project all contribute towards understanding the fundamental processes that occur in continental crust. Among these are earthquake activities and the formation of ore deposits.The primary objectives, therefore, were to gather basic data about the geophysical structure below the KTB site, such as the magnitude and direction of stresses, so that the evolution of the continental crust might be modeled. Information about thermal structure - temperature distribution, heat sources and heat flow- was also needed to understand chemical processes such as the transformation to metamorphic rock and the mineralization of ores. Fluids also play an important role in temperature distribution, heat flow and the various chemical processes, so measurements of pressure, permeability and recovery of fluids found were also important.

The overriding goal of the KTB project was to provide scientist with a permanent, accessible, very deep hole for research. With a budget of 498 million Deustche Marks [ $319 million] - provided by the German government- the initial target was to drill until temperature reached about 300 degree celcius [527 F] -expected at a depth of 10,000 m to 12,000 m [32,800 ft to 39,370 ft] - the estimate limit of borehole technology. This includes drilling hardware, drilling fluid chemistry, cementing as well as the downhole instrumentation required for the various scientific experiments. Many technical spin-offs developed from the project. 


Drilling the Vorbohrung - Pilot Hole

It is not common to drill through sufical crystalline rock especially when the drilling conditions are unknown. Kola SG 3, on the Kola peninsula near Murmansk, Russia, is one exception. It is the world's deepest borehole, but not an ideal role model. After 15 years of drilling, at an untold cost, the borehole reached a depth of 12,066 m [ 39,587 ft]. Years later it was deepened to 12,260 m [ 40,220 ft].

The project management team, having studied the Russian project, decided to first drill a pilot hole - KTB Vorbohrung. This was spudded on September 22, 1987. The objectives for the pilot hole were as follows:

  • Acquire a maximum of geoscientific data, from coring and logging the entire borehole, at low cost and minimum risk before committing to an expensive heavy rig and superdeep hole.
  • Minimize core runs and logging in the large-diameter, straight vertical upper section of the superdeep hole.
  • Analyze the temperature profile for planning the superdeep hole.
  • Obtain data about problem sections with inflow or lost circulation, wellbore instabilities and breakouts.
  • Test drilling techniques and logging tools in preparation for the superdeep hole.

 To accomplish these objectives, a new drilling technique was developed that combined rotary drilling and sandline core retrieval techniques. A modified land rig used a high-speed topdrive to rotate internal and external flush-jointed 5 1/2-in. outside diameter mining drillstring in a 6-in. borehole. This drillstring provided enough clearance inside to allow 4-in. cores to be cut and pulled up to surface through the drillipe by sandline- eliminating round trips to recover cores. A solids-free, highly lubricating mud system had to be used, because of the small clearance between the flush external surface of the drillstring and the borehole wall. This coring method worked well until February 1989 when excessive corrosion in the pipe joints required replacing the mining string with conventional 3 1/2-in. externally upset drillpipe and core  barrels.

Coring operations had to be interrupted on other ocassions - three times for directional drilling to bring the hole back to vertical and twice for sidetracking, because of lost bottomhole assemblies after unsuccessful fishing.However, a total depth (TD) of 4000 m [13,124 ft] was reached for KTB-VB on April 4, 1989, after 560 days of drilling and logging. More importantly, 3594 m [11,790 ft] of cores were recovered a recovery rate comparable to those achieved worldwide in easier formations - and the hole was extensively logged with many different instruments.

The drilling experience in KTB-VB proved invaluable to the planners of the superdeep borehole. For example, they encountered areas of borehole instability across fault zones; they had to modify the mud system to account for water influx and water-sensitive rock; they had numerous breakouts caused by the relaxation of stressed rock; and the formation dipped more steeply than predicted making it difficult to keep the hole anywhere near vertical. In total, the pilot hole presented a greater challenge for drillers than expected. 

For the next year, many experiments and measurements- such as hydrofracs, production tests and extensive seismic work - were carried out in around KTB-VB. In April 1990, the hole was finally cased and cemented. 

Drilling the Hauptbohrung

The superdeep hole- KTB Hauptbohrung (KTB-HB) - was spudded on October 6, 1990, and reached a TD of 9101 m [29,859 ft] on October 21, 1994. To drill to this depth in only four years required the design and construction of the largest land rig in the world- UTB 1. This rig could handle 12,000 m of drillpipe that required a maximum hook load of 8000 kN [1,800,000 lbf] -more than three times that of the rig used to drill KTB-VB. Mechanical wear and tear was expected to correspond to drilling 30 deep conventional wells, with over 600 round trips. Reducing trip time called for radical rig design. Using 40-m [130 ft] stands of drillpipe instead of the standard 27-m [90 ft] stands save 30% of the time. However, long drillpipe stands meant the rig had to be 83 m [272 ft] high.

To further increase trip efficiency, an automated pipe handling system was installed. This consisted of a 53-m [174-ft] high pipehandler that grasped and lifted stands of drillpipe between the rotary table and star-shaped fingerboard for stacking in the derrick. This allowed the pipehandler to operate while the traveling block was moving.




The entire operation was controlled by a driller, pipehandler operator and two floormen. Only the floormen worked outside on the rig floor- the other two sat inside a control room at consoles equipped with video screens and gauge. Computers controlled many of the operations of the pipehandler. Using a pipe conveyor to lift single pipes between rig floor and pipe racks saved additional time.

Borehole torque and drag as well as the strength of the drillpipe are decisive factors when it comes to reaching great depths quickly and safely. Torque while drilling and excessive hook loads when pulling the string are caused by lateral forces and friction between drillstring and borehole wall. These two factors are increased by the weight of drillstring, borehole inclination and severity of any doglegs. Borehole trajectory influences not only drillstring design, but also any proposed casing scheme. A slim-clearance casing scheme cuts down on rock volume drilled, but also requires a near vertical borehole to minimize friction. Without active steering the borehole would start to build angle as was proved in KTB-VB. So KTB commissioned Eastman Christensen- now Baker Hughes Inteq- to develop a self-steering vertical drilling system (VDS). 

The VDS system consisted of a positive displacement motor to drive the drill bit, a battery-powered inclinometer to measure deviation and a hydraulic system to adjust the angle of the drill bit to correct for deviation. Two hydraulic systems were used  : the first system operated external stabilizer ribs that pushed against the borehole wall moving the whole VDS assembly back to the vertical; the second system used internal rams to move the shaft driving the drill bit back to vertical. As long as battery power was maintained to the inclinometer, both systems operated automatically. Inclination, and other parameters such as temperature, voltage and systems pressure, were transmitted to surface by a mud pulser to monitor progress.

The first 292 m [958 ft] of KTB-HB were drilled with a 17 1/2 in. bit and opened up to 28 in. before setting the 24 1/2 in. casing. To meet the requirements of a vertical hole, a 2.5 degree correction to deviation was made as the hole was widened. The next section was drilled with a 17 1/2 in. VDS system to 3000 m [9840 ft] and completed at the end of May 1991. Teething problems with prototype VDS systems meant using packed-hole assemblies (PHAs) during maintenance and repair. Even so, average deviation for this section was less than 0.5 degree. 

The same strategy was used for the 14 3/4 in. hole - alternating between the improving VDS systems and PHAs. A high deviation buildup from 5519 to 5596 m [18,107 to 18,360 ft] during one PHA run led to the borehole being pulled back and a correction made for deviation. The hole continued on course to 6013 m [19,728 ft] where 13 3/8-in. casing was set in April 1992 - horizontal displacement at this stage was less than 10 m [33 ft].

Drilling continued with VDS systems and PHAs and 12 1/4-in. bits. Within this section 45.7 m [150 ft] were cored, including 20.7 m [68 ft] with a newly developed, large-diameter coring system that gave 9 1/4-in. diameter cores. However, in July 1992 at 6760 m [22,179 ft], the bit became stuck. Eventually, after an unsuccessful fishing operation, the hole had to be plugged back to 6461 m [21,198 ft] and sidetracked. In March 1993, over an interval of 6850 to 7300 m [22,474 to 23,950 ft], a major fault system was crossed. The VDS system could not control deviation over this interval and another correction had to be made. This system was thought to be an extension of the main fault that lies along the boundary between sediments to the west and metamorphic rocks to the east- the Franconian line. Along this fault system a displacement of more than 3000 m occured, showing a repetition of drilled rock sequences. This signaled the start of the most difficult drilling yet and additional funds had to be provided by the German government to complete the project - bringing the total cost to $338 million.

At 7490 m [24,573 ft], when the horizontal displacement was only 12 m [39 ft] , the VDS system was abandoned, as borehole temperatures became too high for the electronics. The hole then started to deviate north. 

Within the main fault system the borehole became unstable and more breakouts occured. While tripping out-of-hole stuck at 7523 m [24,682 ft]. Jarring eventually broke the downhole motor housing allowing the pipe to be pulled out but leaving behind a complicated fish. Several attempts to retrieve the fish failed and the hole was finally plugged back to the vertical section - at 7390 m [24,245 ft] - and sidetracked. Drilling again proved difficult and so a 9 5/8-in. liner was set at 7785 m [25,541 ft] in December 1993 to protect this hard-won section of hole.

Difficult drilling continued with a 8 1/2-in. bit down to 8730 m [28,642 ft]. Borehole instability prevented further progress and a 7 5/8-in. liner was set in May 1994. To bypass the unstable section, a sidetrack was made at 8625 m [28,297 ft] through a precut window in the liner. Funds to continue drilling were now running low and a decision was made to stop 476 m [1561 ft] later on October 12, 1994. More than four years after spudding, the hole had reached 9101 m with a final bit of 6 1/2 in. However, the borehole had not finished with the drillers yet. Attempts to lower logging tools into the open hole failed. The last section had to be re-drilled and a 5 1/2 -in. liner set, leaving only 70 m [230 ft] of open hole for the wireline loggers and other scientific experiments. 

Data Collection and Analysis

The main center of scientific activity at KTB was the field laboratory with a staff of 40 including resident scientist and technicians. Here, experiments were performed on cores - mainly from the KTB-VB- drill cuttings and gas traces from the shale shakers, sidewall cores from the Schlumberger Sidewall CoreDriller tool, rock fragments from the drillpipe-conveyed cutting sampler and fluid samples collected during pump test and downhole. 

The field laboratory provided cataloging and storage facilities and a data base of basic information such as petrophysical properties, mineralogy and lithology needed for further experiments.

Nearly 400 logging runs were made in KTB-VB - the pilot hole- with every available borehole instrument. And 266 runs were made in KTB-HB- the superdeep hole. The wealth of data acquired in the field lab allowed a rare opportunity to calibrate borehole log responses to core data in crystalline rocks - as opposed to sedimentary environments where their response is well known- satisfying one of the main objectives of KTB-VB. 

The formations that were cored and drilled consisted of metamorphic basement rocks- principally gneisses and amphibolites. Initially cores and rock fragments - from cuttings- were photographed and cataloged according to depth recovered. Microscopic analysis of thin sections assisted recognition of mineralogy and microstructure and assignment of rock type. By mapping the macroscopic structure and orienting it with borehole logs such as the FMI Fullbore Formaiton MicroImager image or borehole Televiewer (BHTV) image, a structural picture of the borehole was gradually built up.

Petrophysical parameters , such as thermal conductivity , density, electrical conductivity, acoustic impedance, natural radioactivity, natural remanent magnetism and magnetic susceptiblity were also routinely measured. In addition to determining the strength of rock samples, scientists made highly sensitive measurements of expansion of the cores as they relaxed to atmospheric pressure.

Geochemists at the field laboratory performed detailed core analysis using X-ray fluorescence for rock chemical composition and X-ray diffraction for mineralogy. This analysis allowed a reliable reconstruction of the lithology.

After comparing logs with cores,scientists at the Geophysical Institute at the University  of Aachen were able to distinguish 32 distinct electrofacies corresponding to 32 minerals. This enabled borehole logs to contribute to and refine the lithological profile of the superdeep borehole, established from cutting samples and the limited cores available. 

One contributor to the success of the logging operation was the GLT Geochemical Logging Tool. This provided concentrations of 10 elements present in rock: silicon, calcium, iron, titanium, gadolinium, sulfur, aluminum, potassium, uranium and thorium. 

Another tool with a semiconductor detector - germanium- was also used, which gave a higher sensitivity and provided the additional elemental concentrations of sodium, magnesium, manganese, chromium and vanadium. By combining the GLT results with other measurements, minerals such as pyrite, pyrrhotite, magnetite and hematite could be quantified.

Older logging techniques also proved invaluable. Abnormal Spontaneous Potential (SP) deflections occured across mineralized fault systems. Other SP deflections combined with low mud resistivity readings from the Auxiliary Measurement Sonde (AMS) occured at zones of water influx. When the AMS resistivity showed only mud and the SP showed a deflection, this was regarded as an indicator for mineralization. Uranium tends to concentrate at graphite accumulations so the uranium reading from the NGS Natural Gamma Ray Spectrometry tool was used as a graphite indicator.

Although there are several standard high-temperature logging tools available, tools were upgraded especially for KTB. One example is the high-temperature Formation MicroScanner tool, which was upgraded to 260 degree. The first task in modifying this tool was to produce a list of components to upgrade. Several components, such as the pads containing the button electrodes, were not changed, but could be used only once. Other components, such as the hydraulic motor that opens and closes the sonde calipers, could still be used more than once. Mechanical maintenance of such high-temperature tools has to be meticulous- using even one component that should have been changed could result not only in malfunction but also in destruction of expensive equipment.

Temperature limits on the mechanical aspects of the tool were relatively straightforward to overcome. However, the electronics were of major concern. Normally these operate up to 175 degree C. To keep the temperature within this limit meant housing them inside a Dewar flask. The outside temperature could be as high as 260 degree C with the inside remaining below 175 degree C for up to 8 hours.

The cooling effects of mud circulation during drilling were calculated to be about 50 degree C at TD. When circulation stopped, the temperature would gradually climb, giving a window of 36 hours for logging before it exceeded tool ratings. On the first logging run at TD, the maximum temperature recorded was 240 degree C and on the last run , this reached 250.5 degree C- confirming earlier calculations. At the end of each logging run the Dewar flask were cooled down slowly by blowing air through to avoid thermal shock. 



Surprises- Some Welcome, Some Not

Both boreholes yielded unexpected results for the scientists. Geologist had formed a picture of the crust at the Windischechenbach site by examining rock outcrops and two-dimensional (2D) seismic measurements. At a depth of about 7000 m [22,966 ft] they had expected to drill through the boundary between two tectonic plates that collided 320 million years ago, forming the Euarasian plate. However, this boundary was never crossed, and the geologist have had to redraw most of the subsurface picture. 

Other unexpected results include core and log evidence for a network fo conductive pathways through highly resistive rock, and in rock devoid of matrix porosity, an ample supply of water. 



Seismic Investigations 

During the project, surface and borehole seismic measurements helped visualize the structure below the KTB site. The original picture had been formed from 2D seismic work undertaken before drilling. But the structural profile of KTB-VB showed a more complicated subsurface. Instead of a nappe unit, the formation followed a more tortuous path. 

After KTB-VB was completed in April 1989 , a year was spent on major seismic evaluation. The seismic work, under the joint responsibility of KTB and DEKORP - German Continental Reflection Seismic Profiling- was performed by Prakla-Seismos- now part of Geco-Prakla. This included a 3D survey over an area of 19 by 19 km , vertical seismic profile (VSP) and moving source profile (MSP), using geophones in KTB-VB, and two wide-angle 2D seismic surveys with an offset of 30 km using vibrators and explosives as sources. 

The evaluation, conducted by a number of German universities and their geophysical institutes, utilized acoustic impedance calculated from borehole sonic and density measurements and the acoustic measurements mad on cores in the field laboratory. 

The seismic processing was complicated by the tortuous structure and the large seismic anisotropy. The results, however, gave a much clearer picture than the earlier 2D work and accurately predicted the major fault system drilled through between 6850 to 7300 m.

It is known that the borehole remained inside the Zone of Erbendorf Vohenstrauss (ZEV) , a small crystalline unit tectonically placed between the Saxothuringian and Moldanubian units. There are indications that these metamorphic units of the Bohemian Massif have been uplifted 10 km since Variscan time - about 300 million years ago- and eroded to the present day surface.

Future experiments have been designed to measure seismic anisotropy at greater depths, the spatial extension of seismic reflectors- such as the "Erbendorf" structure at a depth of about 12 km - and the detailed velocity distribution between the two boreholes using seismic tomography. Seismologists will also take advantage of the superdeep borehole KTB-HB by recording downhole seismic waveforms emitted by earthquakes. In this way, surface noise will be reduced and the frequency content of the signal preserved.

Electromagnetics- One of the reasons for choosing the Windischeschenbach site was to investigate the origin and nature of a low resistivity layer recorded by surface measurements that appeared to be 10 km below the Earth's surface. This is not unique to southern Germany, as similar layers are found in many continents around the world. 

To unravel the mysteries of this conductive layer, scientists pursued many different angles. Conductivity measurements on cores from KTB-VB showed high resistivity as expected in crystalline rocks. But then highly conductive graphite-bearing faults and cataclastic zones at various depths up to 7000 m. These were also seen on borehole logs where abnormal SP deflections of more than 200 millivolts (mV) coincided with the graphite. Other logs, such induced polarization- where the decay of a voltage applied at a surface electrode is measured downhole- showed conductive pathways potentially formed by veins of graphite and/or sulfides. 

At much larger scale, when the KTB-HB was at a depth of 6013 m a dipole-dipole experiment was carried out. This consisted of using the casing from both holes to inject current into the formation. The resulting potential field was measured around the borehole. Any changes in potential indicated a connection of an electric conductor to one of the casing, supporting the theory for a conducting layer extending over a distance of several hundred meters. The results showed that the conducting layer coincided with graphite deposits in a north-south striking fault system- the Nottersdorf fault zone. The faults from this system crossed KTB-VB at about 250 m [820 ft] and KTB-HB at about 1500 m [4921 ft]. 

Further experiments are planned to investigate the depth, thickness, electrical anisotropy and source of the high conductivity layer still believed to be at 10 km.

Stress and Deformation

One of the goals of earth science is earthquake prediction, and ultimately reduction in earthquake risk. The physics of earthquakes requires an understanding of the movement of tectonic plates, the forces involved and role the crust plays in transmitting those forces. Many scientist think that the top 10 km of crust is brittle and carries most of the stress that moves the entire 100-km thick continental plates. They also believe that, with increasing depth, the crust becomes ductile and cannot support the stress. KTB research may help clarify the transition from brittle to ductile.

Preliminary work in two KTB boreholes has already determined the orientation of the local stress field. The four-arm caliper, resistivity imaging tools, such as the Formation MicroScanner tool , and acoustic imaging tools, such as the BHTV, were used to calculate the stress direction from analysis of two types of failure: shear failure of the borehole wall- called breakouts- and drilling-induced tensile failures. The former occur at an azimuth orthogonal to the orientation of the maximum horizontal stress. The latter are near-vertical fractures in the borehole wall in the direction of the maximum horizontal stress. These fractures were easily identifiable on the cores cut in the KTB-VB and were oriented using Formation MicroScanner and BHTV images. The maximum horizontal stress is oriented to N 150 degree +- 10 degree E from surface down to 6000 m. 

To obtain the stress magnitude, hydrofrac experiments were carried out in both boreholes at various depths in conjuction with geoscientist at the Universities of Bochum, and Karlsruhe, Germany and at Stanford University, California, USA. By fracturing the formation, the minimum and maximum principle stresses were determined.




These and earlier tests in KTB-VB confirmed that the strength of the rock was increasing with depth, supporting the theory that the upper crust is strong enough to carry most of the stress of tectonic movement. Very recently, a hydrofrac experiment was carried out at 9000 m [29,528 ft] and is being evaluated.




 Thermal Studies

Of the many processes occuring within the continental crust, most are temperature dependent. Mapping the temperature distribution and measuring heat production, heat flow and thermal conductivity are therefore a vital part of understanding these processes. During the initial temperature mapping, KTB-VB held the unwelcome surprise that the formation temperature gradient was highger than anticipated. The disappointing result meant that 300 degree celcius - the set limit of current technology- would be reached at about 10,000 m- much shallower than originally predicted.

Temperature measurements were carried out in the two boreholes during regular logging campaigns. These were used to estimate true formation temperature. The borehole is cooled during drilling, by up to 70 degree celcius in the deepest sections of KTB-HB. Formation temperature is obtained by recording several temperature profiles at preset time intervals as the hole heats up again and extrapolating these  profiles to infinite time on a logarithmic plot.

Each temperature profile was recorded during the first wireline logging run. This helped avoid another complication, disturbing the mud temperature profile by the logging tools. A wireline tool was even modified at KTB with the temperature sensor mounted on the bottom of the tool to provide the least disturbance and give the best possible result.

Temperature data provided an opportunity to measure heat production and conductivity. In addition, thermal conductivity measurements were carried out in the field laboratory on cores cut from the boreholes. From the NGS and Litho-Density data, heat produced by radioactive decay was calculated - for metabasites the results were 0.5 micro-Watts per cubic meter.

The final temperature profile has yet to be extrapolated from the data obtained so far. Experiments will continue to examine temperature distribution, heat production, heat flow and thermal conductivity.

Fluids

The scientist at KTB expected deep cyrstalline rock to be bone dry, but to their surprise, water  influx occured at several depths from open fractures.

Sonic, Formation MicroScanner and BHTV data were used to detect the fractures. As fresh mud was  used for drilling, any saline water inflow would cause a decrease in mud resistivity. This could easily be seen from mud resistivity measurements made by the AMS tool. These zones were allowed to produce by dropping the mud level, enabling a fluid sample to be collected by a wireline-conveyed sampler run in combination with the AMS tool. Test showed the water had not leached down recently from surface. Further tests will be performed to ascertain the origin and composition and investigate fluid-rock interaction. 

During a two-month pumping test 275 m^3 [1730 bbl] of salt water were produced from an open fracture system at the bottom of KTB-VB. Further evidence showed the extent of the fluid network. During a produciton test at 6000 m in KTB-HB, the fluid level in KTB-VB dropped. When the 13 3/8-in. casing in KTB-HB was cemented, there was an increase in fluid level in KTB-VB. These two events confirmed hydraulic communication and allowed an estimate of permeability of the fracture system between the two boreholes.

Natural causes of fluid movement became apparent when pressure sensors deployed in KTB-VB rcorded changes in pressure due to earth tides caused by the gravitational pull of the moon.

Fluids play an important role in the chemical and physical processes in the Earth's crust, influencing mineral reactions, rheological properties of rocks and melting and crystallization processes. To aid further scientific research into these processes long-term pumping tests are planned between KTB-HB and KTB-VB to measure hydraulic communication, identify fluid pathways and collect additional fluid samples.